Signal indication devices for indicating a certain underpressure in the air intake pipe of the engine are known. In the case of these devices, underpressure in the air intake pipe and the atmospheric pressure are used in the aforementioned control. The variable control magnitude serves here, thus alone, the underpressure in the air intake pipe, which underpressure changes with the level of contamination of the air filter. The thermal conditions on the engine change, however, also as is known with the speed of the engine. The thermal conditions are much more critical in the higher speed range than in the lower speeds. This means that one and the same engine type operated with lower speeds (for example 15 rotations per minute) can take a substantially smaller pressure than in the case of a continuous operation in the high speed range (for example 3000 rotations per minute). Therefore, a signal device of the aforementioned type cannot be adjusted in advance at its place of manufacture to unknown operating conditions. Therefore, this device is adjusted to the underpressure in the air intake pipe which can be expected at a maximum speed of the engine and it is accepted that during continuous operation of this engine at lower speeds a more frequent air filter change must take place.
The purpose of the invention is to provide here a substantial improvement. According to the invention, the signal device includes an operator, preferably a piston, which influences the signaling device and which is loaded on the one side by the pressure in the air intake pipe and on the other side by the pressure in the storage chamber of the engine. With such a design, the pressure in the storage chamber, which pressure changes with the speed, results as second variable control magnitude, which causes the signal device to also consider, aside from the pressure in the air intake pipe, the speed of the engine or to react to the sensitivity of the engine at high speeds.
The preferred embodiment of the invention is characterized by the piston being stored with its cylinder inside of the air filter and surrounding a hollow screw which serves to secure the air filter in place, inside of which hollow screw is arranged a signal rod which is equipped with a signal head. According to a different characteristic of the invention the chamber on one side of the piston is hereby connected to the inner chamber of the filter, while the chamber on the other side of the piston is line-connected to the inside of the hollow screw which terminates in the storage chamber of the engine.
It is furthermore very advantageous in internal combustion engines having a casing which fully surrounds the engine body, to design the signal head in color and to permit it to project in its signaling position from the upper surface of the casing. Monitoring of the device occurs then at a point on the engine which can easily be overseen by the operator.